A happi (法被, 半被) is a traditional Japanese straight-sleeved coat usually made of indigo or brown cotton and imprinted with a distinctive mon (crest). They are usually worn only to festivals. Originally these represented the crest of a family, as happi were worn by house servants. Later, the coats commonly began to display the crests of shops and organizations. Firefighters in the past also used to wear happi; the symbol on their backs referred to the group with which they were associated. In English, "happi" is most often translated as "happi coat" or "happy coat".
^Drazen, Patrick. Anime explosion!: the what? why? & wow! of Japanese animation. Stone Bridge Press, 2003. ISBN1-880656-72-8. Page 322. "In time, these groups of fire-fighters, adopting uniforms consisting of the short jackets called happi emblazoned with the mon (crest) of the particular group, so that one gang could be distinguished from another."